Radio receiver system



3 Sheets-Sheet 1 P. C. SANDRETTO v RADIO RECEIVER SYSTEM Filgd June 22, 1942 Nov. 14, 1944.

3 Sheets-Sheet 2 P. c, sANDRETTo RADIO RECEIVER SYSTEM Noi-14, 1944.

Filed June 22, 1942 v Nov. 14, 1944.

P. c. SANDRETTQ RADIO RECEIVER SYSTEM Filed June 22, 1942 I 3 sheets-sheet :s

f noise.

Patented Nov. 14, 1944 f/UNITEo STATES PATENT OFFICE Pet'er.,C -Sandretto, Oak Park, Ill., assigner to UnitedrAr Lines, Inc., a corporation of Dela- `ware Application `I une22, 1942, Serial No. 447,901

14 claims. (c1. `25o-20) This`A invention relatesto AradioA receiver systems; more particularly to means `for controlling radioA receivers for communication systems,y and ther invention has for an object the provision of improved control meansof this character.

In modern radio receivers, particularly those `utilizing automatic `volume or gain control'cincuits, the'gain. ofthe receiveris vastly increased when they carrier rof the]v transmitting station ceases. Duringv yperiodsyvhen the transmitter is silent the thermal noise =of the receiveras'wel1 v as any atmospheric noises picked up by it, are

heard with high intensity, In 'communication systems it is the usual practice toy keep the 'carrier of-a transmitter on the air only duringthe Vtime when transmission. isy actually being made in order that ya large number of stations may utilizev the same channel without interference.

` With suchsystems theradio operator must therefore listen to static 'between the periods Whenl transmissions are actually being made.` Under severe atmospheric `conditions the operator is thus vforced to undergo an extremely'unpleasant experience, which is found in many cases to reduce the sensitivity `of the Aoperai'jors ears to the point where it is difficult, if not impossible',

for him to read a transmission when a carrier is turned on. k

As a proposed lsolutionofv thev well recognized problemthus presented, it has been suggested that the gain of the receiver be reduced to the point where the'noise heard bythe operator is no 4longer annoying. This procedure, ofcourse, has the disadvantage that'the gain ofthe receiver is'so greatly reduced yas to render the receiverineiective for the reception of relatively weak stations. It has als'o'fbeen proposed -to `control the audiooutput circuits ofthe receiver by means responsive to the carrier, which means are, intended to ysilence thex-receiveruntil ya carrier-of a certain value is received. Difficulties ariseI in connection ywithI such systems, however, Lbecauseunden severe static conditions the receiver may be turned, on by the peaks ofthe Consequently, when the system is most needed it fails tooperateproperly. Increasing the value lofthe carriery which is required to turn Von the receiver ma'y beweffective .'toylesse'n ,the

extent toy which thev control 'system is susceptible to static, bu'twhen this is done a weak carrier" will no longeroperate the receiver.

Something is yet'to `be desired, therefore, in control systems for communicatior'is receivers, and 'it is a` further .object ofvthis invention'to provide a control system such'that any carrier loud enough to make an impression on the re# ceiver will be effective to'render therreceiver operative, while static conditions, regardless of the intensity thereof, will not' effectbperation' of the control means for the receiver.

In carrying out the invention in vone form, a

standard radio receiver is provided with means for heterodyning selected components of the car- -rier of the incoming signal with selected locally generated frequencies to produce a predetermined audio frequency tone or beat note.` This audio tone or beat note is ltered sharply to eliminate thev static components therefrom, and the ltered tone is utilized to renderthe receiver operative. When no ycarrier-is being received there will be no audio tone or beat note,l and accordingly the receiver will be inoperative `so that static'noisesl will not reach the operator.

For a more complete understanding of the invention reference should now be had to the drawings, in Which: y 'l Fig. 1 is a block diagram illustrating what is V now considered a preferred embodiment of the inventionapplied to a standard superheterodyne radio receiver; and

Figs. 42 to 6, inclusive, are 'similar diagrams illustrating further embodiments Aofv theinvention. f-

In the drawings block diagrams are employed to simplify the showing of the invention, and inasmuch as each of the elements represented by a properly identified and labeled block is a standard device we'll known in the art',- it isnot believed' necessary to illustrate in detail the individual circuit connections thereof.

Referring now to Fig. 1 ofthe drawings, the invention is illustrated vas applied to a `conventional radio receiver havingl a radio frequency Lstages of intermediate frequency amplification indicated in the drawings by the reference numerals I4 and I5, a voice ydetector |6,'and the audio output circuit of the receiver which, in

.theembodiment shown, comprises an audio amplifier I'I, the output conductors I8 of whichv are connected to the contacts I9 of a normally open relay, having an operating winding 20. The

relay contacts are adaptedy to .,beconnected through suitable conductors 2l to utilization means such as a loud speaker or head-phones.

The above-identied elements, of course, constitute the signal channel of a conventional superheterodyne radio receiver, and Vin order to control the output circuit of the receiver so as to render the receiver operative or inoperative, in accordance with the presence or absence of a transmitted carrier in the receiver input, there is provided a control channel comprising an additional intermediate frequency amplifier 22, a frequency converter 23 having associated therewith a plurality of crystal oscillators 24 and 25 for locally generating predetermined frequencies, a suitable band pass lter and amplifier 2B, a detector 21, and a sharply tuned band pass filter and amplifier 28, the output circuit of which is connected through a full wave rectifier 29 to the relay operating winding 20.

The control channel is connected to the signal channel by coupling the intermediate frequency amplifier 22 to the intermediate frequency stages of the signal channel at a point between the intermediate frequency amplifiers I4 and I5. The intermediate frequency amplifier 22 serves to isolate the frequency converter 23 from the signal channel of the receiver, and supp-lies to the frequency converter 23 signal energy having an intermediate frequency component equal to the heterodyne frequency generated in the signal channel. The two crystal oscillators 24 and 25 are so connected to the frequency converter 23 as to provide a double heterodyne action, and the frequency of these two oscillators is so related to the intermediate frequency as to provide sum and difference frequenplurality of sum and difference frequencies, and i this lter amplifier is designed with a sufficiently wide band pass so that when tuned to a frequency having a mean value, as compared to the difference frequencies generated in the converter 2,3, the two difference frequencies will be passed without discrimination. In the detector 21 these two difference frequencies beat together to provide a beat note having a frequency which is determined by the frequency separation of the crystal oscillators 24 and 25, and which is independent of the intermediate frequency derived from the signal channel. Preferably the crystal oscillator frequencies are so selected as to have a very small separation in the audio range, and thus there is produced in the detector 21 an audio beat note of a comparatively lowaudio frequency. This beat note is then filtered in the band pass filter 28 which is sharply tuned so asv to eliminate all other signal components which may be present, including the static side bands.

In the usual receiver the amount of static present in the audio stage is a function of the static side bands that the receiver will accept, and of course the Width of the band of acceptance must be sucient in the conventional receiver to permit acceptance of the audio spectrum, as otherwise the signals will not be understood. In the control channel provided in the above described embodiment of this invention, however, it is unnecessary to'pass audio frequencies other than the predetermined beat note frequency, and consequently there is no limitation imposed on the permissible sharpness of the tuning of the band pass filter 28. The filtered beat note, as previously indicated, is rectified in the rectifier 29, and the rectified current serves to energize the operating winding 2U of the relay.

For purposes of a specific example it will be assumed that the crystal oscillator I3 in the signal channel is arranged to generate a frequency which is less than, and is separated from, the carrier frequency by 385 kilocycles. Consequently there will be produced in the frequency converter I2 heterodyne signals equal to the sum and difference'of the carrier frequency and the locally generated frequency, the difference frequency being equal to 385 kilocycles, and the sum frequency being equal to twice the carrier frequency minus 385 kilocycles, The intermediate frequency ampliers I4, I5, and 22 are so constructed as to pass and amplify this 385 kilocycle intermediate frequency and reject the heterodyne sum frequency. Therefore, a frequency of 385 kilocycles is supplied to the control converter 23 in the control channel.

In order to produce the desired audio beat note for controlling the receiver, the crystal oscillators 24 and 25 are arranged to generate frequencies of 365.5 kilocycles and 364.5 kilocycles, respectively. Although any desired frequencies may be selected, it is desirable to provide a low frequency difference between each of the crystal oscillators and the intermediate frequency in order that relatively low difference frequencies may be generated in the converterl 23. While there will also be generated in the converter 23 sum frequencies equal t0 the sum of the intermediate frequency and the respective oscillator frequencies, only the difference frequencies which are respectively equal to 19.5 and 20.5 kilocycles will be passed by the band pass filter and amplifier 26, which constitutes a 20 kilocycle band pass amplifier so designed as to provide a sufficiently Wide band pass that when tuned to 20 kilocycles it will not disv rcriminate against a 19.5 or a 20.5 kilocycle frequency. In the control detector 21 the 19.5 and 20.5 kilocycle frequencies beat together to form a l-kilocycle note which then passes through the sharply tuned l-kilocycle lter 28, which filter, as heretofore indicated, is electitve to remove the static side bands.

It will now be apparent that whenevera carrier of sufficient intensity to make an impression on the receiver is presentin the signal channel thereof, there will be produced in the control channel a 1- kilocycle beat note which, when rectified, serves to energize the Winding 20 and operate to their closed positions the contacts I9 in the audio cutput circuit of the receiver. In the absence of such a carrier, however, the relay will be deenergized since the control channel will not be effective to produce an audio note capable of passing the l-kilocycle lter 28. The locally generated energy produced by the oscillator I3 cannot pass the intermediate frequency amplifiers I4 and 22, and consequently there is no intermediate frequency input t0 the control converter 23. The locally generated energy produced by the crystal oscillators 24 and 25 will then not produce a heterodyne frequency which can be passed by the ZO-kilocycle band pass lter in amplier 20, and consequently the relay will remain deenergized. The presence of static conditions, regardless of the intensity thereof, is Wholly ineffective to enerthe. signal channel ofthe receiver operative.l f Y In Fig.2- there is represented anotheryembodi'- gizethe relay and isr thus ineiectivey t0 yrender ment of the inventlon'asapplied to a conventional superheterodyne receiver, the signal chan-r nel of which comprises'the same elements I0 to 2l, inclusive, as were previously described in connection with the embodiment of Fig. r -1. In

this case, however,` thecontrol channel is con-v f nected to the signal channel at a pointin the radiotfrequency stage Iso that radio frequency energy frather vthan intermediate vfrequency energy is supplied to the control 'channel.--' In vcir-7v der to produce in the control channel the same l-'kilocyclebeat note, forexarnple, which? was produced in the embodiment-'of Fig.v 1, 'the control channel in Fig. 2 is shown as comprising,`

' quency energy is supplied, and which is coupled to the radio frequency crystal oscillator I3 as Well as to a second radio frequency-crystal oscil- 1ator3l.

The second crystal oscillator I3| A'is adapte'dvto .generate a localffrequenoy different from the frequency of the oscillator I3 by -a fiied value, which f would bel kilocycle,` when it is desired to prof duce a l-kilocycle beat n0te.` `1 'I'hus there will be'y produced in the converter 30 two intermediate frequencies separated from'each other by y1 kilocycle. These twointermediate frequencies, since theyfare separated by only 1 kilocycle, may readilybe' passed by the intermediate frequencyv arnpliers 32 and 33 in the control channel, which amplifiers may beef the conventional type capable of passing v`such frequencies'without appreciably discriminating therebetween'. In the detector 34 these'interr'nediate frequencies beat together to -form` afl-kilocycle beat note which isar'nplified in the audio Aamplifier 35,'f1ltered 'in the sharply tuned4 l-kilocycl'e band pass filter 3i,"and supplied throughthe full Wavere'ctier 31 to the operating winding' 20 of the relay Which controls the output circuit of the signal channel.

` frequencies Will'be produced equal to the sum and difference yof. 200 and 385, and the output circuit of the control converter 40 is coupled to an intermediateffrequency amplifier 42, which is' adapted to pass only the sum frequency of 585 kilocycles. The amplified 585 kilocycle frequency is then-supplied from the amplifier 42 to a control detector 43, and by Way of a suitable connection, represented in Fig. 3 by the conductor y44, there is also supplied to the. control detector 43 the intermediate frequency derived from the signal channel. Thus the 385 kilocycle interl mediate frequency beatswith the 585 kilocycle It will be obviousv that'in this embodiment of the invention, 'as Well asin the previously de scribed embodiment, the frequency` ofthe beat note produced in the control channelv is independent of the'gcarrier frequency of the signal picked up by the; receiver. Consequently frequency swings ,Willnot affect the value of the beat note, and extremely sharp tuning may be y provided at the band passlters 28 and 36.

When no carrier isl present the oscillators I3 and 3l may heterodyne with eachother inthe I converter30 to produce a 1-kilocycle note, but

this note will not be passed and amplified by the intermediate frequency amplifiers 3 2 and 33.

krConsequently the operating"wi nding"20 of the relay'vvill not be energized andther'signal channel of the receiver 'will not be rendered operative to supply *signal energy to the utilizationv means to which the conductorsjZI are connected'.

Inlig. 3 there is l shovvn still anotherembodi- 'ment of the invention comprising a control channelfor producing a suitable l-kilocycle beat note,

for'example, forV the purpose of rendering operafrequency supplied to thedetector 43 tofproduce a difference yfrequency of 200 kilocycles. As shown, the 200'kilocycle output of the control detector 43 is coupled to a second'control converter 45 with which there is also associated a second controll Oscillator 46 adjusted to generate aA local frequency of 20I1kilocycles.. Thusa frequency of 1 .kilocycle` is generated in the second control converter 45, and after passing through a .sharply tuned l-kilocycle band pass filter 41, the l-kilocyclebeat note is rectified in a full Wave rectifier 48 and serves to energize the operating Winding 2B. f i

AIt will be observed that the 200 kilocycle frequencygenerated in the detector 43 is equal to the'frequency of the rst control crystal oscil-y lator 4I and is independent of the frequency of the carrieror the intermediate frequency in the signal channel ofthe receiver. This is due tol the fact that variations in the signal channel intermediatefrequency will cancel out in theportion of the control channel comprising the elements 40, 4I, 42, 43 and the intermediate rfrequency connection 44. Thus, whenever a carrier is` -being supplied to the signal channel of the'receiver there will be produced in the control channel a 1-kilocycle beat note which .will energize the Winding 20 and operate therelay in the signal channel output circuit so as to render the receiver-operative. In the absence of a, carrierthe locally generated frequency supplied by the oscillator I3 Willl be suppressed in the intermediate frequency amplifiers I4 and I5. lThe local frequency generated in the control oscillator 4I Will be suppressed in the inter mediate frequency amplifier 42 and the locally generatedvfrequency of the control oscillator 43 will berejected by the l-kilocycle band pass filter 41. Consequently, the Winding 20 'will not be energized'and the receiver will remain inoperative or' inoperative W.the signalchannel of a con-l *ventional superheterodyne receiver.y The; rel ceiver of Fig. 3 embodies thel sameelements III to 2I, inclusive, as in Figs'.r 1 and 2, 'but in .this L case the control channel is somewhat differentlyv arranged. As shown, the control channel coinprises a converter 40, the input side "of which is tive regardless ofthe static conditions which may exist. Y Y

Referring nowto' the embodiment of the invention yshown in Fig. 4,'thesignal channel and the connected to the intermediate frequency stage of the signal channelfat-a point; between .the

.control channel are combinedthroughout a portion of their extent. In this modiiication, the conventionalfantenna I0 is connected through the usual radio frequency amplifier II to a suitable frequency converter 50,130 which a plurality of radio Ifrequency crystal oscillatorsv 5I and 52 are also connected. `O ne of the crystal oscillators is so designed as to generate a radio frequency differing from the carrier frequency of the receiving signal by an amount equal to the intermediate frequency for which the receiver is designed. The second one of the two oscillators is designed to generate a frequency which differs from the first oscillator frequency by a relatively low audio frequency. yFor example, the frequency difference between the oscillators may be on the order of 31%; kilocycles.

The carrier wave, after being amplified in the radio frequency amplifier H, heterodynes With the locally generated frequencies of the two oscillators to provide two sum frequencies and two difference frequencies separated by the frequency difference of the oscillators. Thus, regardless of any variation in the carrier frequency, the two sets of generated intermediate frequencies will have a constant frequency difference. The two sum frequencies will be suppressed in the intermediate frequency stages of the receiver which comprise two intermediate frequency amplifiers 53 and 54 so constructed as to embody band pass characteristics which will permit both of the intermediate difference frequencies to pass with substantially no discrimination.

These two intermediate frequencies are of course amplified in the intermediate frequency amplifiers and are thereafter demodulated in a suitable .detector 55 wherein the intermediate frequencies beat together to form a 31/2-kilocycle beat note. Thus the audio output of the detector 55 includes in addition to the voice frequencies a constant frequency 3l/z-kilocycle beat note. rlhese audio components are then amplified in a suitable audio amplifier 56 having a plurality of output circuits, which respectively constitute the control channel and the signal channel of the receiver. The control channel comprises 'a 31/2- kilocycle band pass filter 51 which is sharply tuned so as to reject all of the audio components except the l/'z-kilocycle note and also to reject the static side bands which may be present in the output of' the audio amplifier. The output from the 312kilocycle filter 51 is then amplified by`a suitable audio amplifier 58, and, after being rectified in a full wave rectifier 59, is impressed on the operating winding 60 of a suitable control relay. There is of course no limit on the sharpness of the filter 51 because it is unnecessary for any of the other audio components to pass through this filter.

The signal channel connected to the output of the audio amplifier 5B comprises a 3-kilocycle low pass filter 5l, which is effective to reject'the 31/2- kilocycle beat note, but which passes all of the lower frequency audio components. The output of this low pass filter of course constitutes the output circuit of the receiver and is connected through an audio amplifier 51a and suitable output conductors 62 to the movable contacts 63 of.

the output control relay for selectively connecting the signal channel output circuit to suitable utilizing means such as a loud speaker or head- Y phones through suitable conductors 64.

When the operating winding Bil is energized by the rectified 31/2-kilocycle note, the contacts G3 will be closed to render the signal channel of the receiver operative, but in the absence of the 31/2-kilocycle note the relay will remain open to render the receiver inoperative. Of course, in the absence of a carrier the oscillators 5| and 52 will beat together to generate a 31/2-cycle note which will be suppressed by the intermediate frequency amplifiers 53 and 54, and, as in the other embodiments of the invention, static pulses or side bands cannot actuate the control relay because they are filtered out by the sharply tuned band pass filter 51.

While the embodiment shown in Fig. 4 is similar to the previously described embodiments in that the audio beat note generated for control purposes is independent of the carrier frequency and may be sharply filtered to remove all static components, this particular embodiment is sub- J'ect to certain limitations because of the` necessity of providing intermediate band pass filter amplifiers capable of passing both intermediate frequencies, which detracts from the selectivity of the receiver. With this system it is also necessary to limit .the audio respon-se of the receiver sinceonly audio components below 3.5 kilocycles, for example, are passed to the output circuit of the signal channel.

In the embodiment of Fig 5 the above referred to limitation in the modification of' Fig. 4 is overcome, while at the same time retaining a portion of the common signal and control channel. this embodiment the antenna I0 and the radio frequency amplifier Il are similar to the previouslyfdescribed corresponding elements, but the output of the radio frequency amplifier is connected, as shown, to two frequency converters and 66. The frequency converter 65 forms a part of the signal channel of the receiver and is provided with a local oscillator 61 which is adapted to heterodyne with the carrier of the signal energy to provide an intermediate frequency which then passes through two identical intermediate frequency amplifier stages indicated in the drawings by the reference numeral 68. Similarly, the converter 56 forms a part of the control channel and is associatedwith a crystal oscillator 69 having a frequency which differs from the frequency of the oscillator 61 by some suitable audio frequency such, for example, as 15 kilocycles. The intermediate heterodyne frequency producedl in the converter 65 passes through two intermediate frequency amplifier stages represented in the drawings by the reference numeral 10, and as illustrated in the drawings the outputs of the intermediate frequency amplifiers 68 and 18 are connected to a common detector 1|. In this detector the voice frequencies are demodulated and the two intermediate frequencies supplied from the signal channel and the control channel beat together to generate a 15-kilocycle audio beat note. The Voice frequencies and this beat note are then amplified in a suitable audio amplifier 12, the output of which is again divided to provide separate control and signal channels. The control channel comprises a 15-kilocycle band pass filter 13 which serves to reject the Various audio components in the output except for the 15'kilocycle beat note. The filtered beat note is rectified in a suitable rectier 14 and the rectified beat note is impressed `on the operating winding 15 of the output control relay. In the signal channel of the receiver there is provided a l-kilocycle low pass filter 16, which rejects the l-kilocycle tone, but passes all of the audio components below l0 kilocycles. The voice currents passed by the filter 1B are next amplified in a suitable output amplifier 11 which is connected to the movable contacts 18 of the control relay, the co-operating contacts of which are connected through suitable conductors 19 to headphones or loud-speakers, as previously described.

' sponse of the receiver-is not so-,limitedv as ini the embodiment of Fig. 4since all audio frequency components up tolki1ocycle"s will be presenty in the audio output circuit of the signal channel.

It will of course be understood thatmeansOther l than the various outputcontrol relays heretofore described may .be utilized forv renderingthe receiver circuitoperative and inoperative in accordance with the presence or absence of a suitably generated audiobeat note, and in Fig. 6

there isfshown an embodimentof'the invention whionnotonly employs a somewhat dffereht type ofc'o'nt'rol means, but also utilizes a somewhat different control channel for producing the'audio beatnote.Y t

.'Injthe' embodiment of Fig. 6 a conventional heterody'ne receiver similar to the receiver of Fig. 1 Yis employed, consisting rof the saineconventional elements I8 to 16, inclusive, as were'described in` connection With Fig'. l; In this em'- bodiment, however,v the audio amplifier is illustrated'as comprising an amplifying tube 80, the output circuitof which is connected through a A suitable transformer 8|` and conductors 82 toutilization means such as headphones or loud-speakers (not shown). The'control channel comprises an'intermediate frequency amplifier83 which is connectedto the intermediate frequency stage of the'signal channel between the amplifiers IA and I5, and a suitable frequency converter'84 to which is 'connected an oscillator 85. The' oscillator 85 'generatesk "a frequency which differs from` the intermediate frequency of the signal channelby an vaudio, Ifrequency which, for example, maybe 20 kilocycles. "Thus, there" are'produc'edjin'the frequen'cy converter 84 frequencieswhich representthe *sum* and difference `of the intermediate and the oscillator frequencies."

be attenuated 3.27.5 decibels.y Thus if an intermediate frequency of 350,kilocyclesis employed, a total bandwidth of '70 kilocycles will be passed when the suppression'is 32.5 decibels. If, however, a transformer. with ra 'similar Q is used in a 2`0kilocycleband pass filter, a suppression of 332.5 decibelswill prevail with a total band Width of only '.4 kilocycles. With a series of sharply tunedfiilters suchl as arel shown in Fig. 6, it is possible to secure a bandwidth o'f only 2 kilocycles 'with anattenuaticn of'80 decibels. Consequently substantially all of the static components will be' rejected.` As shown in'Fig. 6, the filtered -'kilocycle'n'ote is rectified in a full wave rectifier 81,'. andthe output'voltage of the rectifier is impressed kacrss'a ysuitable resistor 88, a condenserv 89 beingl providedto-'remove any trace of the 20-ki1ocycle note or other frequency from the output of the rectifier.

Because oftheV large number of stages which are includedin'the control channel, thermal effects in the various tubes may produce Afa Vrectified current which appears in the resistor 88.` 'Inordertobalance out this current, a suitable adjustable"resistorj'90 lis provided connected through the" resistor 88 across a suitable source of'high voltage, represented by thefterminal 9| and the ground connection 92; Associatedwith'the resisters" 68 and90, and With the'audio amplifier tube 80 for the purposesl of controlling the out- Iput ofthe receiver, area 'plurality of visolating andl balancing resistors 93,l 91.95.96 and9`1, and

sa' v suitable threeeelement, electric-discharge de- Withv the resistorsv93, 96 and vice for tube 98. S'Iconnected racross theI source of high Voltage, as indicated inthe drawings'and with the grid or controllelement 99 of the' tube 98j connected to the common terminal of the'resistors 8B and 90, the resistor 98Amay be adjusted until the current -flowingthrough the resistors 90 and88 exactly balances the normal current flowing through the resistor -88 due to the thermal eifectof the tubes y hereinb'efore referredy to. Thus, substantially Zero Followingk the frequency converterjBAthere arey disposed in the control' channel a plurality of 20kilocycle band passy lterstand amplifiers 86 which are designed lto pass only the difference rfrequencies and reject' the v ,sum frequencies generated in the converter 84. As lpreviously indicated, the amount of static which appears in the audio output of an ordinaryV receiver isa'function of the static side vbandsthatr the receiverA will accept and the width of this band of acceptance is afunction of Jtwo factors, one kbeingfthataI Width' sufcient to allowacceptance of the audio spectrum must be employed in order that the signals Vmay be understood. This factor, ofcourse, is not present in the control channel ofthe emlbodiment shown in Fig. 6, becausefthere isno necessity for passing audio frequencies other than the 20-kilocyc1e note. The second factor which affects the amount of static lies in the inherent sharpness 'that can be securedwith intermediate frequency amplifiers, this'sharpness being on a percentage basis.l For example, ifthe Q of an intermediate frequency transformer is 200, a frequency 10% `from theacceptance frequency will bias is normallyapplied tofthe gridef the tube 98, which has -a ,high plate current, and consequently a high current vflows through the resistor v9,4.' As shown, this resistor 94 is connected Athrough lthe isolating resistor 95 to the grid fcircuit ofthe amplifier tube and consequently there v*will beapplied to the amplifier 80 a bias sufficiently high to cut off theplate cur.

rent therein and block the` production of audio energy therein. Thus the signal channel of 'the receiver is rendered inoperative-`lv When a carr'erlis receivedjby heterodyned (inthe signal and control channels t sofastof provide a zcgkilocycle nere at the'input terminals` of the rectifier 87, ,a1 voltage is produced across the resistor 88 which effectsa substantial reduction in the plate current of the tube 98 andfthus ldecreases the current flow through f the 'resistor `94. Thus normal bias is restored to Loto 5 rto renderthe receiver ,inoperative in the absence of an audio beat note .inf the control channel, and to render ,the signal `vc lciannel of the receiver' operative whenever a lcarrier is rethe'` receiver and 1 ceived which will enable the production of the desired beat note in the control channel. As in the previously described embodiments, the intermediate amplifiers I4 and 83, in the absence of a suitable carrier, reject the locally generated frequencies produced by the oscillator I3, and the -kilocycle band pass filters 86 in the control channel Will, in the absence of a carrier, reject the locally generated frequency produced by the oscillator 85. This embodiment of the invention, however, differs from the previously described embodiments in that the beat note produced is not independent of the carrier frequency received, and consequently the sharpness of the tuning which may be utilized in the 20-kilocyc1e band pass filters is limited by the accuracy and the constancy of the carrier frequencies to be received. v

While particular embodiments of the invention have been shown, it will be understood, of course, that it is not desired that the invention be limited thereto since many modifications may be made, and it is, therefore, contemplated by the appended claims to cover any such modifications as fall Within the true spirit and scope of the invention.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

l. In a radio receiver having a signal channel including an audio stage; the combination of means for rendering said receiver operative Whenever, and only when, signal energy from a transmitter station is being received and regardless of existing static conditions, comprising; a control channel coupled to said signal channel for energization therefrom; means for heterodyning selected components of the signal energy in said control channel to produce heterodyne signals including a predetermined beat note in the audio frequency range; means in said control channel independent of said signal channel audio stage for sharply filtering said heterodyne signals to pass said audio beat note and substantially eliminate all other components including the static side bands thereof; and means for controlling the output circuit of said receiver to render said output circuit operative or inoperative in response to the presence or absence of said audio beat note at the output side of said lter means.

2. In a radio receiver; the combination of means for rendering said receiver operative Whenever, and only When, signal energy from a transmitter station is being received and regardless of existing static conditions, comprising; means for locally generating at least two signal frequencies differing from each other by a predetermined audio frequency; means for heterodyning said locally generated signal frequencies with selected components of the received signal energy to produce a constant frequency audio beat note regardless of variations in the frequency of the carrier component of the received signal energy; means for sharply filtering the heterodyne signals to pass said audio beat note and reject substantially al1 other components of said heterodyne signals including the static side bands thereof; and means responsive to the presence or absence of said audio beat note at the output side of said filter means for controlling the output of said receiver.

3. In a radio receiver having a signal channel including an audio stage; the combination of means for rendering said receiver operative Whenever, and only when, signal energy from a transmitter station is being received and regardless of existing static conditions, comprising; a

control channel coupled to said signal channel for energization therefrom; means for heterodyning selected components of the signal energy in said control channel to produce-heterodyne signals including a predetermined beat note in the audio frequency range; means in said control channel independent of said signal channel audio stage for sharply filtering said heterodyne signals to pass said audio beat note and substantially eliminate all other components including the static side bands thereof; means for rectifying said audio beat note; and means operable by said rectified beat note for selectively rendering the output circuit of said receiver operative.

4. In a superheterodyne receiver having a signal channel including an audio stage and means for heterodyning the incoming signal energy with locally generated signals to produce intermediate frequency components, the combination of a control channel coupled to said signal channel for energization by said intermediate frequency components, means in said control channel for heterodyning additional locally generated signals with said intermediate frequency components of said signal energy to produce an audio beat note of predetermined frequency, band-pass filter means in said control channel independent of said signal channel audio stage for passing said audio beat note and for rejecting other components of said signal energy including the static side bands thereof, and means responsive to the audio beat note passed by said filter means for rendering said receiver operative only when the input to said receiver includes signal energy components from which said audio beat note may be heterodyned.

5. In a radio receiver having a signal channel terminating in an output circuit, the combination of means for locally generating a plurality of signals having a predetermined frequency difference in the audio range, means for heterodyning said locally generated signals With components of the received signal energy to produce an audio b eat note having a frequency equal to said frequency difference regardless of the frequency of the carrier component of said signal energy, band-pass filter means for passing only said audio beat note and rejecting substantially all other components of said signal energy including the static side bands thereof, and means responsive to said audio beat note passed by said filter means for rendering said signal channel operative to energize said output circuit only when said signal channel includes signal energy components from which said beat ,note may be heterodyned.

6. In a superheterodyne receiver having a signal channel including an audio stage and means for heterodyning the incoming signal energy with locally generated signals to produce intermediate frequency components, the combination of a control channel coupled to said signal channel for energization by said intermediate frequency components, means in said control channel for heterodyning additional locally generated signals with said intermediate frequency components of said incoming signal energy to produce an audio beat note of predetermined frequency, band-pass filter means in said control channel independent of said signal channel audio stage for passing said laudio beat note and for rejecting other 'components vof saidfsignai tenergy including the static side bandsthereof, means for yrectifying said audio beat note', and means responsive to said rectiediaudio beat note for `rrendering said receiver operative"onlywhenthe input to` said receiver includes signalenergy components from which said? audio beat' note may be 'heterodyned. y l 7; In a 'radio receiverv having a signalv channel terminating in fan audio stage coupled toan outpu`t circuit,"the combination of a control channel coupled to said signal channel for energizationy therefrom by a component of the signalen- 'ergy -vin'isaid signal channeLf-means for locally generating signals differing `Vin -frequency from `said signal'energy component, means in said conv trol channelffor f heterodyning i said locally generatedsignals Withsaid signal energy component 'in said control c hannel to produce heterodyne @signals including `an audio beat note, band-pass filter-means in said control :channel independent `of'said signalv channel audio stage for passingv saidr audio beat noteand rejecting' static? side bandscontained in said heterodyne signals, 'and means responsive tosa'id audio beat note for rendering'said signal channel operative to energize said output circuit only when said signal channel' supplies to said 4control channel signalen'- ergy Ycomponents from A whichsaid beat 'note may pbe'heterodyned. i v 8. In a vsuperlreterodyriereceiver having a sig- .ference frequencies therefrom, said last-mentioned difference frequency being an audio beat note equal to the frequency differencebetween said locally generated frequencies, sharpwtuned filter. means for passing said audio beat note andrejecting substantially all other-frequency "fcomponentsf'in'said control channel including the'static' side ybands thereof, and means respon- ,sive to'saidv audio,r beat note -for renderingl said ,receiver operative only when the input to saidl receiver includesv signal .components from which said beat note may be Aheterodyned. l v

l0. In `a superheterodyne receiver having a isignal channel including means for heterodyning -the incoming signal' energy to produce aninter- ,mediate frequency component, the combination of acontrol channel coupled to an intermediate `frequency stage of said signal channel foriener- .gization therefrom, means for locally generating -a first :,frequency, means yfor heterodyning :said rflrstfrequency with the intermediate frequency -component in `said control channelto produce sum; and difference' rheterodyne frequencies,

means 'for amplifyingI one only ofv said sum andl difference frequencies, means` for heterodyning said .amplified frequency With said intermediate frequency component to produce sum and dif- ,ference heterodynefrequencies one of which is equal to said first locally generated frequency,

nal channel including aniaudio stage and means Y for-heterodyning the'incoming signal energy with' locally generated signals` to produce intermediate frequency components, the combination of a control channel coupled to anr intermediate "'frequency stage of said signal channel for energization therefrom, means for locally generating additional signals differing in frequency from said' intermediate frequency components, means in said control channel for heterodyning said `additional locally generated signals with said intermediate frequency components to produce an audio beat note component, band-pass filter means in said control channel independent of Isaid signal channel audio stage for passing said 'audio beat note component and rejecting other components present in said control channel including the static side band components thereof, and means responsive to said audio beat note component forrendering said receiver operative onlyA when the input to said receiver includes signal energy components from which said audio beat note may be produced in said control channel.

9. In a superheterodyne receiver having a signal channel including yrneans for heterodyning the incoming signal energy to produce intermediate frequency components, the combination of a control channel coupled to an intermediate frequency stage of said signal channel for energization therefrom, means-for locally generating a rstfrequency which differs by an audio frequency from the intermediate frequency component inrsaid control channel, means for locally generating. a second frequency which differs by another audio frequency from said intermediate frequency component, means for heterodyning said locally generatedr frequencies with said intermediate frequency component to produce sum and difference frequencies, vaudio frequency band-pass lter means in said control channel forfpassing Without substantial discrimination saidaudio difference frequencies of said heterodyning means, means for heterodyning said audio difference frequencies to produce sum yand difmeansgfor locally generating a second frequency differing froml said firstfrequency by a predetermined audio frequency, means for heterodyning `.said secondlocally generatedy frequency with Asaid last-mentioned sum and difference heterodyne frequencies to produce other heterodyne frequencies one of which is equal to said audio frequency difference, sharp filter means for passingr generated frequencies with the radio frequency component of the incoming signal energy to produce two sets of sum and difference Aheterodyne frequencies, intermediate frequency amplier means for amplifying only the difference frequencies of said two sets of heterodyne frequencies, means for combining and demodulating said amplified intermediate frequencies to produce audio frequency components including a beat y note equal to said audio vfrequency difference, band-pass filter means for passing said audio beat note and rejecting substantially all others of said audio` components including the static 4components therein, low passrlter means for passing audio components of frequencies below said audio beat note, audio output means connected to said low pass lter means,y and means coupled to the output of said band-pass lter means responsive to said audio beat note for rendering said audio output means operative only when the input to said receiver includes a carrier component from which said audio beat note maybe heterodyned.

12. In a radio receiver, the combination of means for locallygenerating a pair of radio frelq uenc1es having a predetermined audio frequency difference, means for' dividing the input signal energy of said receiver into a signal channel and a control channel, means for heterodyning one of said locally generated frequencies with the carrier component of said signal energy in said control channel to produce sum and difference heterodyne frequencies, intermediate fre.

quency amplifier means in said control channel for amplifying only said difference frequency,

means for heterodyning the other of said locally ulating said difference frequencies tov vproduceaudio frequency components including a constant frequency audio beat note equalto said predetermined audio frequency difference, band-pass fil ter means connected to the output of said com mon' means for passing said audio beat note and rejecting from said control channel substantially all others of said audio components including the static components therein, low pass filter means connected to the output of said common means for passing to -said signal channel audio components of frequencies below said audio beat note, audio output means for said signal channel, and means in said control channel responsive to said audio beat note for rendering said audio output means operative only when the input to said receiver includes a carrier component from which said audio beat note may be produced.

13. The method of controllingthe audio output stage of a radio receiver to render said receiver operative only when the carrier of a transmitted signal is being received, which method comprises heterodyning selected components of said carrier with locally generated frequencies to produce heterodyne signals including an audio frequency beat note current, sharply-filtering said heterodyne signals independently of said audio stage of said receiver to eliminate the static components from said signals, and rendering said audio output stage operative or inoperative in accordance with the presence or absence of said beat note current in the output of the filter circuit.

v 14. The method of controlling a radio receiver to render said receiver operative only when the carrier of .a transmitted signal is being received, which method comprises heterodyning selected components of said carrier with a plurality of selected locally generated frequencies to produce heterodyne signals includi-ng an audio beat note current independent of the frequency of said carrier and differing in frequency from said locally generated frequencies, suppressing heterodyne frequencies generated in the absence of said carrier, filtering said heterodyne signals to eliminate static components therefrom, and renderv ing said receiver operative or inoperative in accordance with the presence or absence of said 

